Non-aqueous electrolyte secondary batteries that are small-sized, lightweight, and high-capacity batteries are widely used as power sources for mobile devices. In addition, these batteries have recently been attracting attention as driving power sources for power tools, electric vehicles, and the like. Thus, the batteries are expected to be used in wider applications. Regarding such driving power sources, there is a demand for a higher capacity that allows long operation and enhancement of high-rate discharge characteristics of discharging a high current in relatively short time. In particular, in the applications of power tools, electric vehicles, and the like, it is necessary to achieve a higher capacity while high-rate discharge characteristics are maintained.
A technique for achieving a higher battery capacity is, for example, increasing the charging voltage to thereby widen the available voltage range. However, such an increase in the charging voltage causes an increase in the oxidizing power of the positive electrode active material. In addition, the positive electrode active material contains a transition metal (such as Co, Mn, Ni, or Fe) that functions as a catalyst. Accordingly, for example, a decomposition reaction of the electrolytic solution occurs so that a film inhibiting high-rate discharge is formed on the surface of the positive electrode active material, which is problematic. With consideration of the foregoing, the following have been proposed.
(1) A proposal in which a positive electrode active material containing lanthanum atoms in the surface is used to thereby suppress a decomposition reaction of the electrolytic solution (refer to Patent Literature 1 below).
(2) A proposal in which the surface of a spinel lithium manganese oxide serving as a positive electrode active material is modified with a tungsten-containing oxide to thereby enhance high-rate discharge characteristics (refer to Patent Literature 2 below).